In many applications, it is necessary or desirable to bond together two electrical conductors. For example, it is often necessary to form an electrical connection between a sensing element or device and electrical instrumentation employed for the purpose of interpreting electrical signals from the sensing element device and providing a usable output. In particular, sensing elements such as bi-metal thermocouples, thermosensors including PRTDs and NTs, thermistors, semiconductor diodes, and the like, are often formed by their manufacturers with one or more conductive leads extending outwardly from the sensing element for a relatively short distance, for example, four to ten millimeters. In order to effectively utilize such sensing element, it is necessary to electrically extend the short electrical leads from the sensing element to electrical or electronic instrumentation which could be located a substantial distance away from the sensing element, particularly if the sensing element is located in a harsh or hazardous environment. Typically, an extension cable or wire of the appropriate length is electrically connected on one end to each sensing element lead and on the other end to the instrumentation equipment either directly or through other cables, connectors, networks, or the like. Such extension wires are typically multi-stranded aluminum, copper, or thermocouple alloy wires which are covered with a suitable electrical insulating material.
Many different techniques may be employed for electrically connecting one end of such an extension wire to a lead of a sensing element. Such techniques include resistive welding, brazing, soft soldering, silver soldering, mechanical crimping, laser fusion and ultrasonic welding, sometimes referred to simply as ultrasonic or acoustic welding. Each such technique has its own advantages and disadvantages, none of which are relevant to the present invention which is concerned only with ultrasonic or acoustic welding.
FIG. 1 shows a typical temperature sensing element 10 including a pair of conductive leads 12, 14 extending slightly outwardly therefrom. The leads 12, 14 are typically formed of nickel, copper, gold, platinum, aluminum or some alloy of one or more such elements. As illustrated in FIG. 2, in order to ultrasonically weld a lead 12 to a multi-stranded extension wire 26, either the lead or the extension wire is mechanically retained in place by a capture fixture 44 of an ultrasonic welding apparatus (in FIG. 2 the extension wire 26 is shown on the bottom of the fixture 44 and the lead 12 is shown on top). The other of the lead 12 or extension wire is engaged by an ultrasonic welding head 16 which causes it to vibrate rapidly against the component held by the capture fixture 44 as a result of the application of ultrasonic pulses. The friction of the vibration causes a weld joint to form at the contact surfaces of the lead 12 and the extension wire 26.
While ultrasonic welding as described is suitable in many applications, problems sometimes occur when the sensing element lead 12 has a diameter or lateral dimension which is substantially different from the diameter or lateral dimension of the extension wire 26 to which it is to be welded. In such situations, the generally smaller sensing element lead 12 is not adequately mechanically retained within the capture fixture and is thereby permitted to move laterally or pivotally (shown in phantom in FIG. 2) within the capture fixture 44 during the welding process resulting in an inadequate weld or a weld in which only a relatively small portion of the sensing element lead 12 firmly engages the extension wire 26. Such movement can result in a weakened weld joint as well as diminished electrical conductivity between the sensing element lead 12 and the extension wire 26 sometimes resulting in errors in the output from the electronic instrumentation.
The present invention comprises an improved method of ultrasonically welding together two conductors of different lateral dimensions in which the foregoing problems are avoided. In employing the improved method of the present invention, the capture fixture of the ultrasonic welding apparatus is selected so that the lateral sides of the capture fixture substantially engage both lateral sides of the extension wire. The sensing element lead is formed by bending or crimping so that first and second laterally extending portions are formed. When the sensing element lead is placed in the capture fixture, the first and second portions of the lead substantially engage or are proximate to opposite lateral walls of the fixture to mechanically retain the electrical lead in place within the fixture and to decrease or limit movement of the electrical lead during the ultrasonic welding process. In this manner, an enhanced weld connection is formed between the lead and the extension wire and the area of engagement between the lead and the extension wire is substantially greater than the area of electrical contact would be if the electrical lead were not so formed.